The invention relates to contact lenses for the eye which are made of thermoplastic alloys of cellulose esters of aliphatic carboxylic acids and higher molecular weight aliphatic compounds with ester and/or carbonate groups in the polymer chain, the alloys being essentially free from low molecular weight plasticisers. The invention furthermore relates to the use of such polymer alloys for the production of refracting, semi-hard materials which absorb only small amounts of water, for contact optics.
A number of different plastics are already used in contact ophthalmics. The materials sit directly on the cornea and therefore substantially influence the metabolic processes thereof and the oxygen balance. In addition, irritation effects, for example by contamination and migrating constituents from the plastic, may occur on the eye as a result of the direct contact.
Contact lenses are generally classified into three main types, that is to say into hard, semi-hard and hydrophilic soft lenses. Lenses of the first type have been known for many years and are usually produced from polymethyl methacrylate, which can be cut and polished so that the lenses have good optical properties. However, in practice, hard polymethyl methacrylate lenses have the disadvantage of mechanical irritation to the cornea and the inside of the eyelid. If a patient is to wear hard lenses, this therefore necessitates a considerable acclimatisation period, during which the patient wears the lenses in gradually increasing periods of time, until a maximum period of continuous wear of about eight hours is achieved.
Because of this deficiency of the hard lenses, they are used by only a small percentage of wearers of contact lenses.
Soft contact lenses consisting of hydrogels of hydrophilic polymers have in recent years been introduced into contact ophthalmics. With these lenses, it has been possible to overcome the problem of eye irritation to a great extent, so that patients now become much more quickly accustomed to wearing contact lenses. Although the soft hydrophilic lens materials hitherto used, which are based on hydroxyethyl methacrylate and/or vinylpyrrolidone, initially are more comfortable to wear than hard lenses because of their hydrophilic character, they have the enormous disadvantage that metabolism products are deposited in them, and in addition bacteria, spores or fungi can penetrate and cause an infection of the eye.
There was therefore a need for new refracting materials for contact lenses for the eye, which have neither the disadvantage of deficient comfort during wear of the hard polymethyl methacrylate (PMMA) lens nor the disadvantage of the soft hydrogels based on hydroxyethyl methacrylate (HEMA)/vinylpyrrolidone of easily causing infection. The use of semi-hard materials which absorb only small amounts of water, cause no irritation symptoms, are chemically inert, are very comfortable to wear, accommodate no bacteria, fungi or metabolism products, release no plasticisers and the like and at the same time exhibit only slight light scattering, would be advantageous.
Cellulose acetobutyrate (CAB) is employed as the base material in the semi-hard lenses at present used in practice. The tolerance of CAB lenses is generally better than that of polymethyl methacrylate hard lenses. The reason for this is the better wettability, lower mechanical stress and higher heat conductivity of the CAB lenses. In addition, the transportation of lachrymal fluid is not so greatly inhibited under a hard to semi-hard lens as with a closely fitting soft lens. However, a disadvantage of the CAB lenses is that the materials frequently contain plasticisers and processing auxiliaries, since CAB cannot be processed as a thermoplastic without additives.
Semi-hard to soft materials which substantially fulfil the above requirements have been found in polymer alloys of cellulose esters and polyethylene/co-vinyl acetate (in this context, see DOS (German Published Specification) No. 2,807,663, DOS (German Published Specification) No. 2,856,891 and European Pat. No. 0,013,366 ). However, it has been found that the radii of contact lenses made of such materials may change after prolonged wear; moreover, such materials have a residual content of scattered light, although low, which is to be attributed to distribution of the alloy constituents not being completely homogeneous.
A. The object of the present invention was therefore to discover thermoplastic polymer alloys based on cellulose esters which have comfort in waring which is recognised as good, and the high level of inertness towards bacteria, fungi, metabolism products and the like of the cellulose esterpolyethylene/co-vinyl acetate polymer alloys, but have a smaller residual content of scattered light and give contact lenses with improved stability of radius. Such suitable materials have been found in thermoplastic polymer alloys, which are free from low molecular weight plasticisers, of cellulose esters and certain higher molecular weight aliphatic compounds.
B. The present invention thus relates to contact lenses of an essentially plasticiser-free polymer alloy based on one or more cellulose esters of one or more aliphatic carboxylic acids, which are characterised in that the polymer alloy consists of (A) 97 to 70% by weight, preferably 95 to 85% by weight, of cellulose esters and (B) 3 to 30% by weight, preferably 5 to 15% by weight, of an aliphatic polymeric compound with ester and/or carbonate groups in the polymer chain and a molecular weight of 2,000 to 300,000, preferably 4,000 to 200,000 and particularly preferably 5,000 to 100,000, and contains less than 2% by weight, preferably less than 1% by weight and particularly preferably less than 0.5% by weight, of compounds with a molecular weight below 500, preferably below 1,000. The molecular weights referred to above are number average values (Mn) determined by the gel permeation chromotography.
The semi-hard to soft polymer alloys to be used according to the invention are free from conventional plasticisers and processing stabilisers, without which cellulose esters of aliphatic carboxylic acids usually cannot be processed as thermoplastics without decomposition or discoloration. They are highly transparent to light and have only a very low content of scattered light. It is surprising that, in the mixtures to be employed according to the invention, the two polymers of different refractive index are compatible and no light scattering is exhibited. Incompatibilities and thus cloudiness usually occur with such polymer mixtures.
Cellulose esters which are suitable for the production of the refractive polymer alloys to be used according to the invention are cellulose esters of aliphatic carboxylic acids with 1 to 5 C atoms, preferably cellulose acetate, acetopropionate and acetobutyrate.
Processes for the production of organic cellulose esters have been known for a long time and are described, for example, in Ullmanns Encyclopadie der technischen Chemie (Ullmann's Encyclopaedia of Industrial Chemistry) (Verlag Urban und Schwarzenberg, Munich-Berlin, 1963) in Volume 5 on pages 182 to 201.
Suitable cellulose acetobutyrates contain, for example, 30 to 50% by weight of butyric acid, 10 to 26% by weight of acetic acid and 0.7 to 1.95% by weight of hydroxyl groups.
The use of cellulose acetobutyrates having the following composition is preferred: 35 to 46% by weight of butyric acid, 15 to 21% by weight of acetic acid and 0.8 to 1.7% by weight of hydroxyl groups.
Suitable cellulose acetopropionates contain, for example, 30 to 63.5% by weight of propionic acid, 1 to 12% by weight of acetic acid and 1.2 to 1.95% by weight of hydroxyl groups.
Cellulose acetopropionates having the following composition are preferably used according to the invention: 40 to 60% by weight of propionic acid, 2 to 8% by weight of acetic acid and 1.5 to 1.8% by weight of hydroxyl groups.
The relative viscosities (.eta..sub.rel) of 2% strength solutions, in acetone at 25.degree. C., of the aliphatic cellulose esters used are preferably between 3.5 and 5.0, particularly preferably between 4.0 and 4.5.
Higher molecular weight aliphatic compounds, which cannot be extracted with water, are optionally branched and optionally contain 2 to 8, preferably 2 or 3, and particularly preferably 2, terminal hydroxyl groups, and which generally contain carbonate and/or ester groups and optionally contain ether groups and have an average molecular weight (number-average) of 2,000 to 300,000, are suitable as the second component of the polymer alloys to be used according to the invention. These are preferably polyesters, polyether-esters, polyester-carbonates, polyether-carbonates and polyester-ether-carbonates.